Understanding the Speed of Force: Instantaneous vs. Speed of Light

[PhDorBust]

Is force instantaneous or does it travel at the speed of light, and why?

Like is the gravitational effect felt on us by the sun immediate or does it take 8 minutes or so to be realized?

 

[Integral]

Assuming you are applying a force to something, then the effects of that force travel through the object at the speed of sound in that object. Much slower then the speed of light.

 

[rcgldr]

gravitational effect

wiki:
http://en.wikipedia.org/wiki/Speed_of_gravity

 

[maverick_starstrider]

Is force instantaneous or does it travel at the speed of light, and why?

Like is the gravitational effect felt on us by the sun immediate or does it take 8 minutes or so to be realized?

Gravity's a different creature. But if you were to don a suit of magnetized iron armor and sit on the Earth and send a gigantic electromagnet to the sun and then someone switched it on, then yes, it would take 8 minutes for you to feel the tug.

 

[creepypasta13]

why would it not be instantaneous?

 

[Fuz]

Is force instantaneous or does it travel at the speed of light, and why?

Like is the gravitational effect felt on us by the sun immediate or does it take 8 minutes or so to be realized?

Gravity travels at exactly the speed of light. If the sun were to disappear right now, the Earth would keep revolving as if the sun were still there. After about 8 minutes the Earth would notice the absence of the suns gravity and would just float off into space.

 

[maverick_starstrider]

why would it not be instantaneous?

Because the messenger particles that mediate the force only travel at c.

 

[Gear300]

Mechanical forces propagate energy as mechanical waves (speed depends on the inertial and elastic properties of the the medium). Gravity, by what I've heard and read, propagates at the speed of light - meaning that if the sun suddenly disappeared, the planets that were orbiting it would take a while to go off course.

 

[Phrak]

In this folder gravity is instantaneous. A few folders down it propagates somewhat slower--sort-of.

 

[Pierre007080]

Gravity is instantaneous. If it were not there would be a "drag" factor on any rotating body because the force would be acting behind the radius line. Because mass absorbs space the support between two masses is removed and the effect is instantaneous. If ten matches were separating two masses and two (one from each end) were removed the shortened distance would be immediate. This is the reason for the constant acceleration of a rotating body toward the middle.

 

[D H]

Gravity is instantaneous in Newtonian physics. In general relativity it is not.

Gravity is instantaneous. If it were not there would be a "drag" factor on any rotating body because the force would be acting behind the radius line.

That is a common argument used against general relativity by those who do not understand it. If that is all there was to general relativity that argument would be correct. However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging. For example, for planets far from the Sun and moving at relatively slow speeds compared to light speed it looks very much like gravity does propagate instantaneously. This is not the case for Mercury. That general relativity explained a known defect with the Newtonian explanation of Mercury's orbit was one of the reasons general relativity won the day over Newtonian mechanics.

Because mass absorbs space the support between two masses is removed and the effect is instantaneous.

This is meaningless nonsense.

 

[A.T.]

Gravity is instantaneous. If it were not there would be a "drag" factor on any rotating body because the force would be acting behind the radius line.

As DH already said, this simlistic logic is wrong. It assumes that a field can only contain information about the position of the source during emission. But a field can also contain information about the velocity of the source during emission.

The electric field is like this too: The field lines of a inertially moving charge always point towards the current position of the charge, not some old position from which the field was emitted. But this does not meant the electric field is instantaneous. It just means that the field moves at the same speed as the source was moving during emission.

 

[pgardn]

Isn't the idea of two events happening instantaneously actually a rather bizarre idea in itself? I have read about the idea in quantum mechanics with spins of certain particles, but our basic human model of what instantaneous really is seems ambiguous and hard to define. Just my slightly philisophical take. Time is a human construct. Any ideas that would help me would be appreciated so I thank the first poster for posing the question to begin with. Human models and what reality presents can be vexing. And I really don't know what I mean by reality. anyhow... any particular way people look at events and time would be helpful for me.

 

[russ_watters]

Time is a human construct.

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

 

[maverick_starstrider]

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

The arrow of time may be a human construct.

 

[Gear300]

We can't really be sure whether something is a human construct or not. Even reality, despite how paradoxical it is, is something we take as common sense and for granted.

 

[russ_watters]

The arrow of time may be a human construct.

No, the arrow of time is a consequence of thermodynamics. It is also built into the laws of the universe.

 

[russ_watters]

We can't really be sure whether something is a human construct or not. Even reality, despite how paradoxical it is, is something we take as common sense and for granted.

Yeah, we really can. You guys are arguing against the definition of science here. The whole point of science is to explain/understand natural phenomena. That is - things that exist in nature, whether we are here to observe them or not.

If we define a "year" to be something other than its current definition, that doesn't make the sun any younger or older.

 

[maverick_starstrider]

No, the arrow of time is a consequence of thermodynamics. It is also built into the laws of the universe.

That's not the current consensus I'm afraid. The second law of thermodynamics is not sufficient explanation. Actually Sean Carroll just wrote a book about this (Here to Eternity).

http://en.wikipedia.org/wiki/Arrow_of_time

 

[russ_watters]

? In that wiki article there are 7 "arrows of time" listed and 6 of them are consequences/manifestations of the thermodynamic arrow.

Since that article doesn't mention Sean Carroll, it doesn't really say anything about what you are talking about...

 

[maverick_starstrider]

? In that wiki article there are 7 "arrows of time" listed and 6 of them are consequences/manifestations of the thermodynamic arrow.

Since that article doesn't mention Sean Carroll, it doesn't really say anything about what you are talking about...

Sean Carroll's kind of the Michio Kaku of GR/cosmology if you don't know who he is (http://en.wikipedia.org/wiki/Sean_M._Carroll). Anywho my point merely was that the physicality/reality of a "forward" facing, continuous "arrow of time" is by no means a proved concept in cosmology

 

[Gear300]

Yeah, we really can. You guys are arguing against the definition of science here. The whole point of science is to explain/understand natural phenomena. That is - things that exist in nature, whether we are here to observe them or not.

If we define a "year" to be something other than its current definition, that doesn't make the sun any younger or older.

Couldn't we say that we take for granted what 'natural phenomena' is?

 

[russ_watters]

Couldn't we say that we take for granted what 'natural phenomena' is?

We can say anything, but that doesn't mean it makes any sense... could you explain what you mean?

To perhaps jump ahead, science requires the assumption that our senses or our God aren't screwing with us. Is that what you are getting at?

Applied to time, we measure time based on physical processes such as the oscillations of a cesium atom. Are you saying that if humans weren't here, the number of times a cesium atom oscillated since the sun formed would be different?

 

[maverick_starstrider]

We can say anything, but that doesn't mean it makes any sense... could you explain what you mean?

To perhaps jump ahead, science requires the assumption that our senses or our God aren't screwing with us. Is that what you are getting at?

Applied to time, we measure time based on physical processes such as the oscillations of a cesium atom. Are you saying that if humans weren't here, the number of times a cesium atom oscillated since the sun formed would be different?

Well, if I've correctly dusted the cobwebs from my memory, all quantum systems have time symmetry and anti-particles can be treated as normal particles moving backwards in time with identical results.

 

[Phrak]

We can't really be sure whether something is a human construct or not. Even reality, despite how paradoxical it is, is something we take as common sense and for granted.

Yeah, we really can. You guys are arguing against the definition of science here. The whole point of science is to explain/understand natural phenomena. That is - things that exist in nature, whether we are here to observe them or not.

If we define a "year" to be something other than its current definition, that doesn't make the sun any younger or older.

Rocks are hard and water is wet. The consensus in discussions of physics as a science is that we are assume an independent reality. This is not a know, but an assumption, that we should be using in common.

 

[Gear300]

We can say anything, but that doesn't mean it makes any sense... could you explain what you mean?

To perhaps jump ahead, science requires the assumption that our senses or our God aren't screwing with us. Is that what you are getting at?

Applied to time, we measure time based on physical processes such as the oscillations of a cesium atom. Are you saying that if humans weren't here, the number of times a cesium atom oscillated since the sun formed would be different?

No...I'm just saying things might be like that...then again, even if humans weren't around, the number of oscillations just might be the same. But how do we know which side is true?...and if we were to go even deeper, we probably wouldn't be able to define truth too well either...though I take it that many would say that this goes beyond science. I'm not taking any sides, I'm just saying that both of those options are possible.

 

[maverick_starstrider]

No...I'm just saying things might be like that...then again, even if humans weren't around, the number of oscillations just might be the same. But how do we know which side is true?...and if we were to go even deeper, we probably wouldn't be able to define truth too well either...though I take it that many would say that this goes beyond science. I'm not taking any sides, I'm just saying that both of those options are possible.

This thread is heading for a shift in folders.

 

[Gear300]

This thread is heading for a shift in folders.

Heheh...I noticed.

 

[russ_watters]

Rocks are hard and water is wet. The consensus in discussions of physics as a science is that we are assume an independent reality. This is not a know, but an assumption, that we should be using in common.

So what?

 

[russ_watters]

No...I'm just saying things might be like that...then again, even if humans weren't around, the number of oscillations just might be the same. But how do we know which side is true?...and if we were to go even deeper, we probably wouldn't be able to define truth too well either...though I take it that many would say that this goes beyond science. I'm not taking any sides, I'm just saying that both of those options are possible.

Possible or not, none of that is useful. The fact of the matter is that science, as currently conceived, works. So all this idle speculation about god screwing with us really isn't useful in describing reality.

 

[russ_watters]

This thread is heading for a shift in folders.

To me the problem is simply that people don't believe or accept what science is. Otherwise, such side-discussions wouldn't be necessary. The OP's question wasn't necessarily clear, but the answers to the different possibilities really are.

Even if we were to move it, I don't know where we'd put it because it isn't philosophy, it's just a bunch of wrong assertions about the nature of science and its relationship with reality. These issues really aren't that hard - people just have trust issues when it comes to acceptance of reality.

 

[pgardn]

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

By my reasoning time has no meaning unless events occur.
Time makes no sense to me unless things happen. If all of the sudden
nothing occurred, no events take place, time would be meaningless. Therefore I consider time a human construct. Just as space is. If nothing exists, space has no
meaning. It does not matter if we are around or not. The fact is we are around and
we did make up these words which represent ideas.

I realize the attempts of humans
to understand and predict phenomena rely heavily on ideas or models that
appear to be very accurate in predicting. The struggle to understand our world
must involve some clarification in order to be sure assumptions we have made
are sound. I was under the assumption that simultaneous, which is a word that
represents an idea, must be carefully defined. I thought one had to be very
careful in using the idea that an event that occurs on the face of the Earth
and an event that occurs at the "edge" of the universe can occur simultaneously.

Any further clarification would help me out. I know this is probably too philisophical,
and maybe not practical to many, but I get fairly confused thinking we are not all
on the same page about ideas such as simultaneous.

 

[maverick_starstrider]

By my reasoning time has no meaning unless events occur.
Time makes no sense to me unless things happen. If all of the sudden
nothing occurred, no events take place, time would be meaningless. Therefore I consider time a human construct. Just as space is. If nothing exists, space has no
meaning. It does not matter if we are around or not. The fact is we are around and
we did make up these words which represent ideas.

I realize the attempts of humans
to understand and predict phenomena rely heavily on ideas or models that
appear to be very accurate in predicting. The struggle to understand our world
must involve some clarification in order to be sure assumptions we have made
are sound. I was under the assumption that simultaneous, which is a word that
represents an idea, must be carefully defined. I thought one had to be very
careful in using the idea that an event that occurs on the face of the Earth
and an event that occurs at the "edge" of the universe can occur simultaneously.

Any further clarification would help me out. I know this is probably too philisophical,
and maybe not practical to many, but I get fairly confused thinking we are not all
on the same page about ideas such as simultaneous.

Well simultaneity, as you describe, is just a matter of SR and really doesn't, by any stretch, have to engage the "edge of the universe" (if such a thing exists). GPS satellites have to use SR (and GR apparently), and it's observed everyday in accelerators around the world. The picture of simultaneity it paints, however, is very well understood. It may defy common intuition but it has an extraordinarly accurate scientific and experimental model. Those kind of questions, for lack of a better word, are "child's play" in the pantheon of physics.

 

[pgardn]

Well simultaneity, as you describe, is just a matter of SR and really doesn't, by any stretch, have to engage the "edge of the universe" (if such a thing exists). GPS satellites have to use SR (and GR apparently), and it's observed everyday in accelerators around the world. The picture of simultaneity it paints, however, is very well understood. It may defy common intuition but it has an extraordinarly accurate scientific and experimental model. Those kind of questions, for lack of a better word, are "child's play" in the pantheon of physics.

I used the "edge of the universe" to magnify what my point is. I still don't know what it means. Or if it is even useful to know what it is supposed to mean. I understand the practical uses in classical physics. But beyond that, simultaneity does not make sense to me. I need to go back and look at those funky particles that were together and separated, and then the spin (or someother measurable quantity) was changed in one and the other supposedly changed immediately or simultaneously. This has big implications for me on the way I view this idea and the propigation of "information" requiring no time thus we think events can take place simultaneously. I think that was what I was reading about. ANd I got rather blown away...

 

[Pierre007080]

As DH already said, this simlistic logic is wrong. It assumes that a field can only contain information about the position of the source during emission. But a field can also contain information about the velocity of the source during emission.

The electric field is like this too: The field lines of a inertially moving charge always point towards the current position of the charge, not some old position from which the field was emitted. But this does not meant the electric field is instantaneous. It just means that the field moves at the same speed as the source was moving during emission.

I do not think that electric and gravitational fields should be used as a comparison because the speed of light is and the photon are not involved with gravity. The graviton is (I think) a theoretical particle which still does not solve our problem.

 

[Pierre007080]

Gravity is instantaneous in Newtonian physics. In general relativity it is not.


That is a common argument used against general relativity by those who do not understand it. If that is all there was to general relativity that argument would be correct. However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging. For example, for planets far from the Sun and moving at relatively slow speeds compared to light speed it looks very much like gravity does propagate instantaneously. This is not the case for Mercury. That general relativity explained a known defect with the Newtonian explanation of Mercury's orbit was one of the reasons general relativity won the day over Newtonian mechanics.


Please explain which aspect of General Relativity has to do with gravity being intstantaneous. Frame dragging has other explanations, has it not?

 

[D H]

Please explain which aspect of General Relativity has to do with gravity being intstantaneous. Frame dragging has other explanations, has it not?

I never said it did. Gravity has a non-zero propagation time in general relativity. Specifically, the speed of light.

I did make a mistake in my previous post. I said "However, there is more (a whole lot more) to general relativity than a finite propagation time for gravity. Other terms such as frame dragging nearly cancel the effects of frame dragging."

That should read "However, there is more (a whole lot more) to general relativity than a finite propagation speed for gravity. Other terms such as frame dragging nearly cancel the effects of this finite propagation speed."

Sorry for any confusion.


There are competing non-crackpot theories of gravity. They all have a finite propagation speed for gravity. They all have something analogous to frame dragging. Most are a lot more complex than general relativity and have lots of magic numbers (tuning parameters). General relativity has but one magic number, G. Until some experimental result shows that general relativity is wrong on some account, it wins by virtue of being the simplest theory that matches known facts.

 

[A.T.]

I do not think that electric and gravitational fields should be used as a comparison

I did not say that they are the same. I just gave you an example of a field which propagates at a finite speed and yet pulls towards the current position of a inertially moving source, and not the position where it was emitted. Just because an attractive field was emitted at a certain position, it does not necessarily mean that the it still pulls towards that position at a certain distance.

 

[VincentJL]

No it isn't. It is a human word used to describe a natural phenomena. Time exists whether we are here to observe it or not.

This is not a scientific claim. Though you may hold this belief yourself, it is no more supported by evidence than the existence of an Intelligent Designer. There is a fine line between science and philosophy, and one should tread lightly when close to it

 

[russ_watters]

It is a starting assumption upon which science must be based and all of our theories depend.

 

[D H]

This is not a scientific claim.

I disagree. Russ' claim, "Time exists whether we are here to observe it or not," is very much a scientific claim. It touches upon the core motivation for science: That there exists a material, physical world out there that operates by some set of rules independent of us. Scientists are not solipsists.

 

[VincentJL]

Science is not a religion, and is very much based on the notion that all models are wrong, but some are usefull. You could state simply that science does not claim that which it cannot prove.

To argue that the world IS indeed in one way or another is, no matter which way you would argue, a philosophical discussion.
It's true, that science wouldn't be of much use if indeed all the models where wrong enough to be unusefull, but this does not change the fact, that simply because a model is usefull, it should be considered true. Which would also seem a very stupid notion, given the history of science, where one "True" theory after another have fallen pray to falsification when new phenomena was discovered.

Whether the statement that "Time exists whether we are here to observe it or not" is true or false is not something any scientist should spend his time trying to answer, the more important question is, if we assume this as a model, what are the consequences, and are these such that we can falsify the model. If we cannot falsify the model, then we examine what usage we can find with model. Of cause in the end, if the model is without uses, and cannot be falsified, then it is simply rubbish (even if true).

But of cause this debate has already digressed, I simply though I would add to it, that the subject is of cause philosophy and not science, and debating philosophy often brings very little reward with it.

 

[D H]

Science is not a religion, and is very much based on the notion that all models are wrong, but some are usefull.

I disagree. I'll keep the discussion to physics, but the same discussion applies to other sciences.

Up until the turn of the 20^th century, physicists thought their models were absolutely correct. The statement that "science is very much based on the notion that all models are wrong" is historically false. Except for the few physicists who aspire to the most austere interpretation of "shut up and calculate," it is not true today. For one thing, that would make theoretical physicists little more than glorified engineers. For another, physicists would not spend time debating which is the correct interpretation of quantum mechanics if "science is very much based on the notion that all models are wrong". Finally, even if that statement were true, that does not mean that the truth is not out there. The fact is that the vast majority of physicists still believe the truth is out there -- and it is their job to find it, or at least a better approximation of the truth.

The view that "all models are wrong" is a mischaracterization of how science works. Popper's philosophy of science is a bit naive. That physicists and engineers still use Newtonian mechanics to this day falsifies Popper's philosophy of science. Newtonian mechanics has not been falsified; it has instead been shown to be of limited (but very useful) domain. Suppose some physicist or team of physicists come up with a Theory of Everything. This will not falsify quantum mechanics and general relativity. It will instead show that QM and GR are themselves of limited domain.

 

[QuantumPion]

...For one thing, that would make theoretical physicists little more than glorified engineers.

Hey...what you you have against engineers?

 

[D H]

Me personally? Nothing. I've been working as an engineer for the last thirty years. Scientists apparently do; I was more-or-less parroting words from the recent Copenhagen interpretation thread.

 

[Winter Flower]

Because the messenger particles that mediate the force only travel at c.

Hi. Can someone explain more about the theories that say forces are
mediated by particles? So is it that the type of particle determines
the speed of propigation?

For electromagnitism the particles are photons, which move at c and elecromagnetic waves = light which also propigates at c?

 

[russ_watters]

It is a starting assumption upon which science must be based and all of our theories depend.

While what I and DH said is correct here, it is actually too weak and incomplete.

The idea that the laws of the universe are consistent throughout the universe isn't just a starting postulate for the whole of science, it is a testable theory as well. It is contained explicitly in Special Relativity as a postulate, but by testing the theory, we test the postulate. Put another way: Every time a new experiment is performed or a new observation is made in space, this fundamental postulate is tested and the theory that the laws of the universe are consistent throughout the universe gains a higher level of proof. At this point, the possible room for this postulate to not be true is very, very small.

 

[russ_watters]

Just to add to what DH said:

Which would also seem a very stupid notion, given the history of science, where one "True" theory after another have fallen pray to falsification when new phenomena was discovered.

Science is the endless pursuit of the "True" laws of the universe. That individual scientists have been confident that their theory is that True law is not a failure of science - it isn't even a failure of human nature. It is the personal belief/confidence by a scientist that they are capable of finding the True laws that drives them to look. And the self correcting nature of the scientific process encourages - even requires - scientists to make errors for the purpose of learning from them.

Note that you are no longer arguing that True laws don't exist, but only that we haven't found them yet or aren't capable of finding them. This is, of course, irrelevant to whether or not they exist and you are now providing an argument that requires that they exist.

Whether the statement that "Time exists whether we are here to observe it or not" is true or false is not something any scientist should spend his time trying to answer...

Indeed any scientist who investigates the nature of time is investigating precisely that question.

But of cause this debate has already digressed, I simply though I would add to it, that the subject is of cause philosophy and not science, and debating philosophy often brings very little reward with it.

Though it has digressed, sometimes we find (particularly with the subject of time) that people don't have a clear understanding of what the point of all this is. So it is good to remind them sometimes.

 

[pgardn]

It is also fairly obvious to me that humans can only understand and perceive a certain amount of what might be present in the universe. We are limited by our evolutionary history. We are not infallible machines. We use metaphor and analogy to try and model some things that don't lend themselves to our senses built on everyday experience. We use machines to turn events we cannot perceive into light and sound... we are highly visual animals that might be missing a heck of a lot. The tremendous invention of logic and math help us predict and further more lead us to look for things we might have never have expected to even exist. In fact our "common sense" based on the world we sense directly (which play a major role in the proper development of our brain) can do us a diservice in some cases. I don't mean to sound like Plato, that observation creates confusion, but I have to be humbled by how limited we are. And amazed how clever some people can be.

And I still don't understand how some particles separated can apparently "inform" one another of the state they are in simultaneously. I don't get this at all. I may be misinterpreting what is a very difficult idea... Its the Bell stuff and entangled particles. The experiments done with this stuff seem to have consequences that are apparently consistent with quantum mechanics but seem to violate some rules I may have wrongly attributed to the idea of simultaneous events. Maybe this is entirely the wrong section to bring this up, but the original post and responses thereafter got me thinking about this again. I am led to the belief we have to be careful how we think about time based on some of the experiments done. Or maybe I am just entirely missing something.

 

[Gear300]

It is also fairly obvious to me that humans can only understand and perceive a certain amount of what might be present in the universe. We are limited by our evolutionary history. We are not infallible machines. We use metaphor and analogy to try and model some things that don't lend themselves to our senses built on everyday experience. We use machines to turn events we cannot perceive into light and sound... we are highly visual animals that might be missing a heck of a lot. The tremendous invention of logic and math help us predict and further more lead us to look for things we might have never have expected to even exist. In fact our "common sense" based on the world we sense directly (which play a major role in the proper development of our brain) can do us a diservice in some cases. I don't mean to sound like Plato, that observation creates confusion, but I have to be humbled by how limited we are. And amazed how clever some people can be.

And I still don't understand how some particles separated can apparently "inform" one another of the state they are in simultaneously. I don't get this at all. I may be misinterpreting what is a very difficult idea... Its the Bell stuff and entangled particles. The experiments done with this stuff seem to have consequences that are apparently consistent with quantum mechanics but seem to violate some rules I may have wrongly attributed to the idea of simultaneous events. Maybe this is entirely the wrong section to bring this up, but the original post and responses thereafter got me thinking about this again. I am led to the belief we have to be careful how we think about time based on some of the experiments done. Or maybe I am just entirely missing something.

Common sense is not necessarily truth, whereas even truth is not necessarily truth...though I guess we do have to be more careful when thinking scientifically.